A technique for planarizing build surfaces in three-dimensional printing includes the use of a three-dimensional printer having an extruder with a sensor mechanically coupled thereto, where the sensor is operable to sense a contact force between the extruder and a separate structure. In particular, the sensor may be used to measure the contact force at a plurality of locations across a build surface of the separate structure, so that a difference between the measured contact force at two or more locations can be identified. In response to the difference between the measured contact force at these locations, a control signal may be created to reduce the difference for planarizing the build surface, e.g., by fabricating a layer on the build surface that mitigates irregularities therein, by gap filling on the build surface, or by adjusting a parameter of the three-dimensional printer.